PROCESS FOR MAKING HIGHLY CROSSLINKED POLYVINYLPYRROLIDONE

Abstract: A process is described for making highly crosslinked polyvinylpyrrolidone (PVP) polymer by popcorn polymerization having a swell volume of less than about 65 ml/10 g, preferably 40-65 ml/10 g, in the absence of added crosslinking agent, and with substantially no induction period. The process comprises (a) providing a reaction mixture of vinylpyrrolidone (VP) monomer at a concentration of about 75-85 % in an aqueous basic solution, containing about 0.4-0.8 % base, under agitation, in an inert gas atmosphere, and at an initial gas pressure of at least 2 bars, (b) heating said mixture at a predetermined temperature and time sufficient to generate an active crosslinker in situ, and (c) reducing the temperature of the mixture to a temperature whereupon popcorn polymerization of said VP monomer commences substantially immediately to form the desired crosslinked PVP.

Description: PROCESS FOR MAKING HIGHLY CROSSLINKED POLYVINYLPYRROLIDONE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to highly crosslinked polyvinylpyrrolidone (PVP) polymer and, more particularly, to a process for making PVP which has low swell volume and high absorptive capacity properties by polymerization of vinylpyrrolidone (VP) monomer in the absence of added crosslinker. 2. Description of the Prior Art Highly crosslinked or insoluble PVP, known as crospovidone, has been made by popcorn or proliferous polymerization of vinylpyrrolidone, as described in U.S. Patents 3,277,066; 3,306,886; 3,759,880; 3,933,766; and 3,992,562. The process of U.S. Patents 3,277,066 and 3,306,886 involved charging VP in an autoclave and heating at about 140°C. in the presence of an aqueous caustic solution to generate a crosslinker intermediate in situ (stage 1), cooling the mixture to about 100°C, and holding for a sufficient period of time, known as the "induction time", for onset of popcorn polymerization to occur (stage 2) . The polymerization is continued until substantially complete conversion of monomer has occurred (stage 3) . However, this 3-stage process is a time- consuming process, particularly because of the considerable length of the induction period. Furthermore, the PVP product obtained had a relatively high swell volume and low absorptive capacity properties. In U.S. Patents 3,759,880; 3,933,766; and 3,992,562, VP was polymerized to crosslinked PVP by heating an aqueous solution of VP at 100°C. in the presence of an added bifunctional crosslinking agent. However, this process is run at a relatively low VP solids level, which is uneconomical. Accordingly, it is an object of this invention is to provide a process for making highly crosslinked polyvinylpyrrolidone by popcorn polymerization of vinylpyrrolidone having a low swell volume, suitably less than 65 ml/10 g, preferably 40-65 ml/10 g, in the absence of added crosslinker. Another object herein is to provide popcorn polymerization process for making highly crosslinked PVP in which there is substantially no induction period. These and other objects and features of the invention will be made apparent hereinafter. SUMMARY OF THE INVENTION A process is provided herein for making highly crosslinked PVP by popcorn polymerization having a swell volume of less than about 65 ml/10 g, preferably 40-65 ml/10 g, in the absence of added crosslinking agent, and with substantially no induction period. The process comprises: (a) providing a reaction mixture of VP monomer at a weight concentration of about 75-85% in an aqueous basic solution, containing about 0.4-0.8% base, under agitation, in an inert gas atmosphere, and at an initial gas pressure of at least 2 bars, (b) heating said mixture at a predetermined temperature and time sufficient to generate an active crosslinker in situ, and, (c) reducing the temperature of the mixture to a temperature whereupon popcorn polymerization of said VP monomer commences substantially immediately to form the desired crosslinked PVP. IN THE DRAWINGS FIGURE 1 is a graphical representation of temperature vs. time profile for the 3-stage polymerization process according to U.S. Patents 3,277,066 and 3,306,886. FIGURE 2 is a similar graphical representation of temperature vs. time for the process steps of the present invention. FIGURE 3 is a contour plot of the operation parameters of base content vs. initial gas pressure in the process of the invention showing the effect of added pressure on the induction time of the process, and the swell volume of the crosslinked PVP product. DETAILED DESCRIPTION OF THE INVENTION Referring now to FIGURE 1, there is shown a graphical representation of the temperature vs. time profile for the popcorn polymerization of VP of the prior art, e.g. as described in U.S. Patents 3,277,066 and 3,306,886. In this process, a reaction mixture of VP in an aqueous caustic solution is placed in a autoclave reactor, such as a Buchi reactor, under the following reaction conditions: Monomer cone. - 80% by wt. NaOH content - 0.4% Initial pressure of reaction mixture - 0.14 bars The reaction mixture is kept under a minimum positive pressure of a nitrogen gas atmosphere in order to provide an inert gas atmosphere over the reactants. The mixture then is agitated at 814 rpm and heated to about 140°C. This period is referred to as (A) in Figure 1. The heated mixture then is held at this temperature for a sufficient period of time to generate an active crosslinker in situ (stage 1 of the process) . This period requires about 4 hours in the process. The temperature of the reaction mixture containing the crosslinker then is reduced to about 100°C. (during B) and held at this lower temperature until onset of polymerization occurs (the induction period) . Typically, as shown, the induction period is about 1-1/2 hours (stage 2) . The polymerization step itself (stage 3) then is spontaneous and follows immediately after the induction period. The mixture then is allowed to cool towards room temperature (C) . Concurrently, with changes in temperature during the process, the pressure of the reactants developed in the autoclave rises and falls from an initial value of 0.14 bars to a maximum of about 4 bars at 140°C. to 0 bars after polymerization. The total time required for this process is about 7-1/4 hours. In the invention process, illustrated in FIGURE 2, the following reaction mixture: Monomer cone. - 82% NaOH content - 0.57% Initial External N 2 pressure - 2.0 bars FIGURE 3 shows a contour plot of several parameters in the process illustrating the experimental conditions necessary to achieve the desired attributes of substantially no induction period during the process, i.e. less than 20 minutes, and a swell volume of crosslinked PVP product of less than 65 ml/10 g, preferably 40-65 ml/10 g. These results are obtained at an initial external gas pressure of at least 2.0 bars, preferably 2.4-3.0 bars, at a caustic concentration of about 0.4-0.8%, for a VP monomer concentration of about 75-85%. The total time for the process shown is about 4-3/4 hours; subsequent examples were carried out with stage 1 (crosslinker generation) times of 2-1/2 hours providing even shorter process times. While the invention has been described with particular reference to certain embodiments thereof, it will be understood that changes and modifications may be made which are within the skill of the art. Accordingly, it is intended to be bound only by the following claims, in which:

Claims: WHAT IS CLAIMED IS: 1. A process for making highly crosslinked PVP by popcorn polymerization having a swell volume of less than about 65 ml/10 g, preferably 40-65 ml/10 g, in the absence of added crosslinking agent, and with substantially no induction period, which comprises: (a) providing a reaction mixture of VP monomer at a weight concentration of about 75-85% in an aqueous basic solution, containing about 0.4-0.8% base, under agitation, in an inert gas atmosphere, under an initial gas pressure of at least 2 bars, (b) heating said mixture at a predetermined temperature and time sufficient to generate an active crosslinker in situ, and (c) reducing the temperature of the mixture to a temperature whereupon popcorn polymerization of said VP monomer commences substantially immediately to form the desired crosslinked PVP. 2. A process according to claim 1 wherein, in (a), said initial gas pressure is about 2.4-3.0 bars. 3. A process according to claim 1 wherein, in (a), said base content is about 0.4-0.6%. 4. A process according to claim 1 wherein, in (b) , said predetermined temperature is about 140°C, and, in (c) , said polymerization temperature is about 100°C. 5. A process according to claim 1 wherein the crosslinked PVP product has a swell volume of about 45 to 65 ml/10 g. 6. A process according to claim 1 wherein, in (b) , said period of time is about 4 to 4-1/2 hours.

Inventors: TSENG SUSAN Y, TSENG, SUSAN, Y.
Applicants: ISP INVESTMENTS INC, ISP INVESTMENTS INC.
Application year: 1994
Main classification: C08F226
IPC codes: C08F26